Conventional golf balls can be divided into two general types or groups: solid balls and wound balls. The difference in play characteristics resulting from these different types of constructions can be quite significant. Balls having a solid construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. One-piece solid golf balls are well-known in the art. Other types of solid balls, also well-known in the art, are made with a solid core, usually formed of a crosslinked rubber, which is encased by a cover material. Typically, solid cores are formed of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. The solid cores are covered by a tough, cut-proof cover, generally formed of a material such as SURLYN.RTM., an ionomer resin produced by E.I. DuPont de Nemours and Co. of Wilmington, Del.
Ionomer resins have, to a large extent, replaced balata as a cover material. Chemically, ionomer resins are a copolymer of an olefin and an .alpha.,.beta.-ethylenically-unsaturated carboxylic acid having 10-90% of the carboxylic acid groups neutralized by a metal ion. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid neutralized with metal salts, such as SURLYN.RTM., and IOTEK.RTM. from Exxon Corporation. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
Surrounding the core with an ionomeric cover material provides a ball that is virtually indestructible by golfers. Further, such a combination imparts a high initial velocity to the ball which results in improved distance. Because the materials of which the ball is formed are very rigid, solid balls generally have a hard "feel" when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate which is another factor in providing greater distance.
It is well known in the art to modify the properties of a conventional solid ball by altering the typical single layer core and single cover layer construction to provide a multi-layer ball having such as a dual cover layer, dual core layer and/or a ball having a mantle layer disposed between the cover and the core. Like the solid cores, various cover layers, mantle layers, and intermediate layers are typically formed of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. The playing characteristics of multi-layer balls, such as "feel" and compression, can be tailored by varying the properties of one or more of these intermediate layers.
The wound ball, though, remains the preferred ball of more advanced players due to its spin and feel characteristics. Wound balls typically have either a solid rubber or liquid center core around which tensioned elastic thread or yarn are wound. This wound core is then covered with a durable cover material, such as SURLYN.RTM., or a softer cover material, such as balata or polyurethane. Balata is a natural or synthetic trans-polyisoprene rubber that is the favored cover material for highly-skilled golfers because of its softness. Wound balls are generally softer and provide more spin than non-wound balls, a characteristic that enables a skilled golfer to have more precise control over golf ball direction, flight profile, and distance. The enhanced control and stopping ability of a balata-covered, wound ball is particularly evident on approach shots into the green, where the high spin rate of these balls enables the golfer to stop the ball very near its landing position. Balata-covered balls, however, are easily damaged, and therefore lack the durability required by the recreational golfer.
A number of patents have been issued that are directed towards modifying the properties of layers used in forming conventional solid balls, multi-layer balls having dual cover layers, dual core layers and/or balls having a mantle layer disposed between the cover and the core, and wound balls. For example, polyurethanes have been recognized as useful materials for golf ball covers since as early as about 1960. U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover.
Polyurethane is the product of a reaction between a polyurethane prepolymer and a curing agent. The polyurethane prepolymer is a product formed by a reaction between a polyol and a diisocyanate. The curing agents used previously are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Polyurethanes are typically divided into two categories: thermosets and thermoplastics. Thermoplastic polyurethanes are formed by the reaction of a diisocyanate, such as 4,4'-diphenylmethane diisocyanate ("MDI") or 3,3'-dimethyl-4,4'-biphenylene diisocyanate ("TODI"), and a polyol cured with a diol, such as 1,4-butanediol. Thermoset polyurethanes are formed by the reaction of a diisocyanate, such as 2,4-toluene diisocyanate ("TDI") or methylene-bis-(4-cyclohexyl isocyanate) ("HMDI"), and a polyol which is cured with a polyamine, a triol such as trimethylol propane, or a tetrafunctional glycol, such as N,N,N',N'-tetra-bis-(2-hydroxypropyl)ethylenediamine.
Since 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a diamine. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent and/or a difunctional glycol. The first commercially successful polyurethane covered golf ball was the Titleist.RTM. Professional ball, first released in 1993.
Unlike SURLYN.RTM. covered golf balls, polyurethane golf ball covers can be formulated to possess the soft "feel" of balata covered golf balls. However, golf ball covers made from polyurethane have not, to date, fully matched SURLYN.RTM. golf balls with respect to resilience or the rebound of the golf ball cover, which is a function of the initial velocity of a golf ball after impact with a golf club.
U.S. Pat. No. 3,989,568 discloses a three-component system employing either one or two polyurethane prepolymers and one or two polyol or fast-reacting diamine curing agents. The reactants chosen for the system must have different rates of reactions within two or more competing reactions.
U.S. Pat. No. 4,123,061 discloses a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a fast-reacting diamine curing agent.
U.S. Pat. No. 5,334,673 discloses a golf ball cover made from a composition of a thermosetting polyurethane prepolymer and a slow-reacting polyamine curing agent and/or a difunctional glycol. Resultant golf balls are found to have improved shear resistance and cut resistance compared to covers made from balata or SURLYN.RTM..
U.S. Pat. No. 5,692,974 discloses methods of using cationic ionomers in golf ball cover compositions. Additionally, the patent relates to golf balls having covers and cores incorporating urethane ionomers. Improved resiliency and initial velocity are achieved by the addition of an alkylating agent such as t-butyl-chloride which induces ionic interactions in the polyurethane to produce cationic type ionomers.
International Patent Application WO 98/37929 discloses a composition for golf ball covers that comprises a blend of a diisocyanate/polyol prepolymer and a curing agent comprising a blend of a slow-reacting diamine and a fast-reacting diamine. Improved "feel", playability, and durability characteristics are exhibited.
Conventional polyurethane elastomers are known to have lower resiliency than SURLYN.RTM. and other ionomer resins. It has now been discovered that the use of a polyurethane composition, according to the present invention, in forming golf ball cores, intermediate and mantle layers, and/or covers, can raise the velocity of a golf ball prepared with the composition closer to the velocities observed with SURLYN.RTM.-covered golf balls. Additionally, the curatives used in the compositions of the present invention are less expensive than many curatives typically used by those of ordinary skill in the art, such as VERSALINK.RTM. P-250, an oligomeric diamine manufactured by Air Products and Chemicals, Inc. Moreover, the compositions of the present invention are less environmentally hazardous than other cost-effective options, such as ETHACURE.RTM. 300 and ETHACURE.RTM. 100.